Many medicament delivery devices that are out on the market for self-administering of doses of medicament are arranged with dose setting features. These may be parts of the medicament delivery device that is operable in relation to other parts, such as dose drums that are rotated in relation to a housing. In some solutions the different housing parts are moved in the longitudinal direction in relation to each other when a dose is set. For instance, the document U.S. Pat. No. 5,226,896 discloses an injection pen comprising a collar and a syringe housing in threaded engagement with each other. In order to set a dose of medicament, the syringe housing is rotated in relation to the collar, whereby the syringe housing is moved inside the collar, making the housing of the device shorter. On the other hand, when setting a dose, a distal end of the device comprising a cap attached to a distal end of a plunger rod is extended in the distal direction. This is because the proximal end of the plunger rod is in contact with a stopper in a syringe filled with medicament, and when the syringe housing is moved in the distal direction, so does the stopper, plunger rod and cap due to the incompressibility of the medicament in the syringe. Thus, the device retains more or less the same length during the setting of a dose. When then an injection is to be performed, the cap with its plunger rod is pressed linearly in the proximal direction, causing a dose delivery. Further, the pressure on the medicament in the syringe during dose setting may cause a leakage of medicament when an injection needle is attached to the proximal end of the syringe.
Document WO 2006/130098 displays a medicament delivery device comprising a proximal cartridge housing comprising a cartridge. The proximal housing part is threadedly connected to a distal back cover provided with a dose indication drum. For setting a dose of medicament, the back cover is rotated in relation to the proximal cartridge housing. When the back cover is rotated, a plunger rod spring is compressed. Further, a servo spring in the form of a clock spring is also arranged to aid the plunger rod in the injection operation, and is tensioned when the back cover is rotated. In order to deliver a dose, the proximal end is pressed against and injection site, whereby the plunger rod spring and the servo spring are released by a needle shield affecting a release mechanism, which springs force the plunger rod in the proximal direction, whereby a dose of medicament is delivered.
The device according to WO 2006/130098 is rather bulky and difficult to handle and is provided with double springs which makes a somewhat complicated device in particular if the device is to be used as a disposable medicament delivery device.
Regarding device size and functional features, especially regarding disposable medicament delivery devices, there are further developments to be made.